The invention relates to an integral, asymmetric, synthetic separating membrane for the nanofiltration range and the lower ultrafiltration range based on polymers, which are soluble in .epsilon.-caprolactam, a process for production of the same, and the application of these membranes.
Membranes produced from synthetic polymers have been known for a long time. The use of .epsilon.-caprolactam as a solvent in the production of membranes made of synthetic polymers is already a recognised method.
DE-PS 3 327 638 describes a method for the production of porous shaped articles, in which a suitable hollow fiber is manufactured from polyamide-6, .epsilon.-caprolactam and polyethylene glycol. Shaping takes place at a nozzle temperature of 210.degree. C. The spinning solution is homogenous and has a low viscosity and must therefore be extruded into a U-shaped cooling tube, in which the mechanical load exerted on the polymer mixture is kept low until the point where solidification begins.
According to the described method, precipitation of the polymer takes place by a thermally induced process. Coagulation diffusively induced by a precipitation agent does not effectively take place. The membranes described in DE-PS 3 327 638 are suitable for microfiltration, and generally have an isotropic structure.
An indication is also given that it is possible to obtain an anisotropic system of pores, but except for the comment that there is a gradient inside the shaped article in the direction of the surface, no further information is provided about the asymmetry of the membranes.
EP-B1-0 361 085 describes integral, asymmetric polyether sulfone membranes, methods for their production and their use in ultrafiltration and microfiltration. The polyether sulfone membranes mentioned here have maximum pore diameters within a range of 0.02 to 2 .mu.m, so that these membranes are mainly suitable for microfiltration and for the upper, large-pore ultrafiltration range. Membranes which are suitable for nanofiltration, haemodialysis, haemodiafiltration and haemofiltration and the lower small-pore range of ultrafiltration are not described in this patent specification.
EP-B1-0 357 021 describes a method for the production of membranes from certain polymers, in which .epsilon.-caprolactam is used as the main solvent component and in which the shaping of membranes and other articles is conducted on the principle of phase separation. The membranes described in this patent specification are also used in the microfiltration and ultrafiltration ranges, and are also suitable for controlled drug release.
It is well-known that membranes which are intended for use in certain separating purposes must also fulfill certain requirements. Their function is to allow exchange processes, whereby this may, for example, be to remove solid particles from a liquid or to separate dissolved particles, depending on the allocated task.
It has been found useful to divide the separating processes into certain categories, whereby the range of reverse osmosis is referred to as hyperfiltration. As the pore size increases, the next range up is nanofiltration, followed by ultrafiltration, microfiltration and particle filtration.
This division into five different filtration ranges has proven successful in practice, but it is important to note that the ranges may overlap at their upper and lower ends.
In the particle filtration range it is relatively easy to place the pore size in relation to the permeability and retention capability for particles of a certain size, since in these ranges both the particle size and the pore size can be determined relatively easily, e.g., they can be seen with the naked eye, at least in the upper range of particle filtration, and with optical microscopes in the medium and lower ranges. In these filtration ranges, the particles to be separated are solid particles which essentially maintain their geometric dimensions during filtration. This also applies essentially for the range of microfiltration, in which very fine particles such as color pigments, bacteria, soot particles in tobacco steam, etc. can be filtered out. Here, it is still possible to place the pore size and particle size in relation to each other.
For ultrafiltration membranes, which have narrower pores, the cut-off of the membrane is determined. In this process, precisely defined solutions of molecules with a known molecular weight, size and shape are used under defined filtration conditions. Measurements using aqueous polydisperse dextran solutions are usually undertaken, which make it possible to determine the cut-off of the membrane for a wide range of molecular weights. This method is described, for example, in Biotechnology, Vol. 9, pages 941-946, year of issue 1991 (G. Tkacik and S. Michaels).
It is common practice to determine the sieve coefficient of cytochrome C, albumin and other proteins of a defined molecular weight, especially for medical dialysis membranes (artificial kidney). The sieving coefficient is defined as ##EQU1## whereby C.sub.Permeate is the concentration of the substance to be determined in the filtrate (the permeate) and C.sub.Parent solution is the concentration of the substance in the original solution.
In the context of the present invention, the upper range of ultrafiltration is the range in which the membrane pores which determine the cut-off have a diameter of 0.02 .mu.m and above. The lower range is the ultrafiltration range in which the membrane pores which determine the cut-off range have a diameter of less than 0.02 .mu.m.